In recent years, development of electronic devices such as display devices, image sensors, and the like provided with a photosensor part that uses a TFD has been in progress.
Patent Document 1 discloses an image sensor in which a photosensor part that uses a TFD and a driver circuit that uses a thin film transistor (TFT) are provided on the same substrate. Patent Document 2 discloses a display device provided with an optical touch panel. This display device can detect that an object such as a finger is in contact with a display surface using external light by providing photosensor parts that use TFDs in a display region of a liquid crystal display device. Patent Document 3 discloses a display device provided with an optical touch panel that detects that an object such as a finger is in contact with a display surface by using not only external light incident upon the display panel, but also reflected light initially radiated from a backlight. Patent Document 4 proposes a display device provided with an optical touch panel that uses a sensor that detects invisible light such as infrared light in order to conduct sensing using invisible light emitted from a backlight.
In display devices and image sensors such as those described above, a TFD semiconductor layer and a TFT semiconductor layer are preferably formed using the same semiconductor film. In order to achieve desired TFT characteristics, it is preferable that a crystalline semiconductor film be used for the semiconductor film, and for example, a low temperature polysilicon film is used. The thickness of the low temperature polysilicon film is approximately 20 to 70 nm, for example. In such a case, because the TFD semiconductor layer (low temperature polysilicon layer) is thin, the TFD semiconductor layer cannot absorb a sufficient amount of relatively long wavelength light (infrared light, for example), which means that there is a possibility that a high sensitivity cannot be attained. For example, when attempting to absorb substantially all of the infrared light with a wavelength of 850 nm, the thickness of the TFD semiconductor layer needs to be approximately 10 μm to 20 μm.
However, when making the semiconductor layer made of low temperature polysilicon thicker, there is a possibility that productivity is reduced because the crystallization process such as laser crystallization conducted on an amorphous silicon film requires longer time.
In particular, when conducting crystallization of the amorphous silicon film using a catalyst element, it is difficult to make the crystalline silicon film thick as stated above. In this crystallization process, after the amorphous silicon film with a catalyst element added thereto undergoes solid-phase crystallization through annealing, for example, a laser is radiated onto the crystallized film, thus further increasing the crystalline quality thereof. In order to obtain a crystalline silicon film with excellent crystalline qualities in the crystallization by annealing and the subsequent laser irradiation step, it is necessary to keep the thickness of the crystalline silicon film at no greater than approximately 50 nm, for example.
Thus, in the past, it was difficult to achieve desired TFT characteristics while ensuring the desired sensitivity of photosensors that use TFDs when a TFT and a TFD were formed using the same semiconductor film.
To address this problem, Patent Document 5 proposes a display device provided with a reflective material layer between a TFD semiconductor layer and a substrate. In the display device of Patent Document 5, a concavity is formed in a region of a substrate surface where a TFD is to be formed. A reflective material layer is formed along this concavity and a TFD semiconductor layer is disposed above the reflective material layer with an insulating film therebetween. With this configuration, light reflected by the reflective material layer can be guided onto the TFD semiconductor layer, and thus, the sensitivity (effective sensitivity) of the TFD to long wavelength light can be improved.